Sensing Resistor in Buck-Boost controllers

Question

In the LT8390, there are two sensing resistors. The one connected to LSN/LSP is called RSENSE(Buck), and the one connected to ISP/ISN is called RSENSE(BOOST). To illustrate this more clearly, I've provided a basic schematic:

^{simulate this circuit – Schematic created using CircuitLab}

My question specifically pertains to RSENSE(BOOST). Based on my understanding (which may be incorrect), there are two different methods to determine its value:

1st method:The maximum fixed voltage value around RSENSE(BOOST) is given as 100mV at full range. For example, if we want the maximum current to be 5A, this would suggest RSENSE(BOOST) to be around 20 m ohms.

2nd method:Using equations on page 20 of the datasheet:

This formula suggests that what passes through this resistor is not IOUT but rather the switch current. The switch current in boost is given by:

Thus, it suggests that RSENSE(BOOST) is calculated as 50mV (not 100 mV anymore because 50mV is in peak-boost) divided by the switch current.

I have two questions:

1. Which method provides the correct sensing resistor value?

2. If I'm only using this buck-boost as a buck (Vin=28V, Vout=20V), I'm not in a fully buck condition, but the formulas also don't make sense anymore. Saying that because delta (IL) does not make sense any more. What exactly does this sensing resistor do in this buck scenario? And how to calculate it in this scenario?

Answer 1

In the LT8390, there are two sensing resistors. The one connected to LSN/LSP is called RSENSE(Buck), and the one connected to ISP/ISN is called RSENSE(BOOST).

This is wrong. There's only one RSENSE connected to the LT8390: The one going across LSP and LSN (inductor current sensing). The value of this resistor is calculated based on the peak buck / boost inductor current. In particular, the datasheet you linked states on page 20:

The final RSENSE value should be lower than the calculated RSENSE in both buck and boost regions. A 20% to 30% margin is usually recommended. Always choose a low ESL current sense resistor.

There is no separate "RSENSE(BOOST)" and "RSENSE(BUCK)" resistor. These are just intermediate results for the RSENSE calculation.

The resistor you're referring to (the one connected between ISP and ISN) is actually called RIS, as shown in this figure from the datasheet:

RIS sets the overcurrent protection threshold. The LT8390 trips off (or goes into hiccup mode) when the DC voltage across this resistor exceeds 100mV. So if you want the LT8390 to go into a fault state at 5A, its value should be 20 mOhms. In practice, you'd set the overcurrent threshold somewhat higher than the maximum expected output current (i.e. 8A for a 5A nominal output).

Note also that there's supposed to be a capacitor "in front" of RIS, which means that only DC (with minor ripple) is flowing through it.